Can knowledge provide its own justification? This sceptical challenge - known as the "problem of the criterion" - is one of the major issues in the history of epistemology, and this volume provides its first comprehensive study, in a span of time that goes from Sextus Empiricus to Quine. After an essential introduction to the notions of knowledge and of philosophy of knowledge, the book provides a detailed reconstruction of the history of the problem. There follows a conceptual analysis of its logical features, and a comparative examination of a phenomenology of solutions that have been suggested in the course of the history of philosophy in order to overcome it, from Descartes to Popper. In this context, an indirect approach to the problem of the criterion is defended as the most successful strategy against the sceptical challenge.